Why i got the test different S11 with different ground?

[John_li]

Hi all,
I measured 5.8G helix antenna(S11 parameter) soldered with different gounds as photoes.And I got the different results,i do not know what cause the different results,anyone can help me?


Thank you

 

[FvM]

At 5.8 GHz, your feeding cable to PCB interface can be expected to have already a considerable reflection coefficient. It can't be identical for the three cases.

The cable position also affects the antenna impedance to some extent.

Generally, the effects could be better understood from a S11 frequency sweep, it would reveal e. g. resonances.

 

[John_li]

Thank you for reply.And any criteria to obey?and how to select the correct point ?Any theoretics to explain the differnce?